The template synthesis and X-ray characterization of pyrrole-derived hexadentate uranyl(VI) Schiff-base macrocyclic complexes

Abstract

The template condensation of 3,4-diethyl-2,5-dicarbaldehyde with ethylenediamine and 1,3-propanediamine in the presence of UO 2(NO 3) 2 gave neutral complexes of formulae UO 2(C 24H 32N 6) ([UO 2(bi-pyen)], 2) and UO 2(C 26H 36N 6) ([UO 2(bi-pytmd)], 3), respectively. X-Ray diffraction quality single crystals were obtained from chloroform/methanol. In the case of 2, these were monoclinic, space group C2/ c, with a = 30.549(5), b = 8.251(1), c= 21.969(4) A ̊ , β = 114.47(1)°, V = 5040(1) A ̊ 3, Z = 8, ϱ calc , = 1.78 g cm −3 . Crystals of 3 were also monoclinic, space group C2/ c, with a = 26.946(7), b = 9.436(2), c = 10.862(2) A ̊ , β = 105.04(2)°, V = 2667(1) A ̊ 3, Z = 4, ϱ calc = 1.75 g cm −3 . In the X-ray structure of 3, the complex lies on a crystallographic twofold axis. For 2, the final R = 0.0335, wR = 0.0371 for 297 parameters and 4475 reflections [ F ⩾ 4( σ( F oo ))], while for 3, the final R = 0.0223, wR = 0.0265 for 213 parameters and 2810 reflections [ F ⩾4 ( σ( F o ))]. In each complex, the uranyl(VI) cation is bound to all six nitrogen atoms such that the metal center, with its two apical oxygen atoms, lies in a distorted hexagonal bipyramidal environment. The change in overall macrocycle size that results in going from 2 to 3 does not affect the UN bond lengths. Rather, the principal difference between 2 and 3 is the exten tent of the twist between the two nearly planar halves of the macrocycle. In 2, the imine-pyrrole-imine dihedral angle is 17.5(1)° while in 3 it is 35.7(1)°. On the other hand, it is near 0° in an earlier reported complex, 1, derived from o-phenylenediamine [J.L. Sessler, T.D. Mody and V. Lynch, Inorg. Chem., 31 (1992) 531]. Complex 1 displays two reversible oxidations at +0.82 V and +1.09 V versus F c F c + in CH 2Cl 2 containing TBAPF 6, while complexes 2 and 3 display irreversible oxidation waves at +1.22 V, +1.46 V and +1.22 V and +1.47 V versus F c F c + , respectively. Complexes 1, 2, and 3 also display quasi-reversible uranyl-centered reductions at −0.85 V, −1.02 V, and −0.96 V versus F c F c + , respectively.